Hierarchical data has always been a challenge for relational database management systems (RDBMSs) because a relational model is flat while hierarchies exhibit inherent recursive properties. Nevertheless, many business operations feature hierarchical data. For example, business users need to manage human resource (HR) hierarchies, which model the relationships between employees. Queries on these hierarchies may retrieve all employees who work under a certain supervisor, or may determine whether an employee A is supervised by an employee B. Conventionally, such a query is modeled as a recursive SQL query operating on a parent column. This approach is inefficient.